Materials are known to present different color characteristics when exposed to a spectrum of ambient light, including ultra-violet (UV) light. Commonly used printing materials, such as paper, present elevated effects in terms of differences in color presentation and/or perception when exposed to UV light, at least in part due to fluorescence brighteners introduced into the paper during the manufacturing process.
Optical brighteners typically absorb light in the UV wavelength range of 320 to 410 nm and re-emit fluorescence light in the visual blue spectral range between 420 to 550 nm. The maximum of the fluorescence spectrum lies between 430 and 440 nm. The reflection of the fluorescence light in the visual blue range impacts human's color perception for purposes of color printed on paper, particularly paper with brighteners involved. The effect of brighteners in printing materials also varies largely from one product to another.
On the other hand, the UV component of lighting varies widely in the day-to-day ambient environment. For example, viewing conditions vary widely for outdoor environments, indoor environments, under different weather conditions, and based on different lighting conditions/light sources. Together, such variables create endless possibilities as to how much UV light is presented to a particular substrate, e.g., paper/printing material.
The combination of the above factors leads to non-linear behavior of how a printing material presents color characteristics when exposed to different lighting. As a result, problems are created for color reading, color measurement and color matching when fluorescing elements are present, e.g., in printing materials. Many manufacturers introduce “blockers” in the printing paper in an effort to block or dampen the abnormal spectral ‘blue’ behavior. The introduction of blockers has also resulted in high demands and introduced significant challenges with respect to color measurement technology.
In addition, UV light is not easily characterized by technologies and instruments in the moderate price range, since light wavelengths of below 350 nm need to be accurately detected and characterized.
In terms of currently available technologies, color measurements may be made with the so called “bi-spectral” measurement method. A bi-spectral measurement device generally includes a monochromatic in the illumination optics and a spectral analyzer in the receiver channel, with the measurement carried out sequentially. A complete reflection spectrum is measured for each illumination wavelength and stored in the form of a matrix. The resulting reflection spectrum of the sample is determined by multiplication of the matrix with a vector which represents the spectral optical energy distribution of the demanded light type. Examples of commercially available bi-spectral measurement systems are the BFC-450 device available from Labsphere, Inc. (North Sutton, N.H.) and the CM-3800 device available from Konica Minolta Holdings, Inc. (Osaka, Japan).
Bi-spectral measurement technology is generally effective. However, the sequential measurement course required to effectuate the bi-spectral technique is time consuming. Realization and/or implementation of this measurement technology is also relatively expensive, putting it beyond the reach of most industrial applications.
With reference to the patent literature, U.S. Pat. No. 6,844,931 to Peter Ehbets describes a color measurement system with variable light emitting diode (LED) illumination and a spectral analyzer in the receiver. The LED light source consists of a multiplicity of differently colored, white and UV LEDs. The individual LEDs can be individually controlled so that the spectral illumination distribution can be electronically adapted to the desired spectrum. The determination of the spectral reflection factor of the sample is then carried out with a single measurement with the desired illumination spectrum.
U.S. Patent Publication No. 2007/0086009 A1 of Peter Ehbets et al. describes a solution for finding the response to different levels of UV by using separately controllable LEDs. For color measurement of samples printed on a substrate including a brightener, a raw spectral reflection factor of the sample is measured in a first measurement by illumination of the sample with light without UV portion. In a second measurement, a fluorescence spectrum of the sample is measured by illumination of the sample with only UV light. The measured fluorescence spectrum is recalculated as a corrected fluorescence spectrum by weighting with spectrally dependent correction factors and, finally, the measured raw spectral reflection factor and the corrected fluorescence spectrum are added to form a corrected spectral reflection factor from which the values characterizing the color of the sample are then calculated. The spectral correction factors are determined during the device manufacture for a certain set of light types and stored in the device. [See FIG. 2 herein for schematic illustration.]
The color measurement instrument/technology described in the Ehbets '009 publication has certain limitations. In particular, such design does not solve the problem of determining and/or addressing how much UV light is present in the light source with which a substrate/printing material is being viewed.
Challenges encountered in color reading, color measurement and/or color matching are illustrated by the following example. In particular, as demonstrated in FIG. 1, two patches that match under one light may not match under another light because of fluorescence. To illustrate this point, a physical patch #1 may be printed on paper with paper brightener. The measured reflectance of patch #1 is shown in the top left portion of FIG. 1 when illuminated by Light #1 that includes no UV light.
A second physical patch—patch #2—is printed on paper without paper brightener. The measured reflectance of patch #2 when illuminated by Light #1 (no UV light) is shown in the top right portion of FIG. 1.
The colors of patch #1 and patch #2 are the same when converted to the colorimetric XYZ color space. In addition, a human observer would not be able to discern a significant color difference between the two patches when illuminated with Light #1.
With further reference to FIG. 1, the lower portion of such figure provides color measurement data for the same physical color patches, but viewed under a light source that includes UV light. Thus, reflectance data for patch #1 and patch #2 are measured with UV-included illumination are provided in the lower portion of FIG. 1. So measured, the color patches are not the same when converted to the calorimetric XYZ color space. In addition and in contrast to the patches when illuminated with a light source that omits UV light, a human observer would be able to discern a significant color difference between the two patches.
The color-related issues illustrated with reference to FIG. 1 arise in many contexts. Thus, for example, color-related issues arise when printing onto paper that includes paper brightener(s) that fluoresce in response to UV light. This fluorescence changes the perceived color of the paper and much of what is printed on the paper. Thus, it may be unknown how a printed color will appear under a particular light source.
The interplay between optical brightener, paper and UV wavelength is illustrated in FIG. 3. With reference to the upper plot of FIG. 3, optical brightener efficiency as a function of wavelength is shown for three different types of paper. Paper #1 responds to wavelengths from 300 nm to 420 nm. Paper #3 only responds to wavelengths between 380 nm and 420 nm. With reference to the lower plot of FIG. 3, the properties of two different light sources are shown. Light source #1 peaks at around 320 nm, whereas light source #2 falls off gradually from 420 nm.
Based on the two plots of FIG. 3, it is apparent that (i) Paper #1 will be excited by both light sources, (ii) Paper #2 will be excited by both light sources, but much more by light #2, and (iii) Paper #3 will not be excited by light source #1, but will be excited by light source #2. As is readily apparent from the foregoing illustration, measuring colors with one UV light source will not capture the entire pattern of excitation. In addition, measurement of the UV level of a light source alone is also insufficient.
It is an object of the present invention to provide systems and methods for improving the precision of color measurement and color matching, particularly in view of the potential for different light sources and/or lighting conditions having varying levels of UV light. It is a further object of the present invention to provide systems and methods that may be simply, reliably and cost efficiently implemented relative to known color measurement and/or color matching processes.
These and other objects are satisfied by the systems and methods disclosed herein.